It seems to have so far escaped OSNews' notice (if the top few hits for a site-search for 'Steam' is any indication) that Steam for Linux is now in Open Beta; you can get the Linux steam client from steampowered.com. So far, they appear to only be making an Ubuntu .deb available, and the client will require closed-source GPU drivers in order to work.

@WorknMan: There are plenty of F/OSS engines out there that support Linux, say iD's, yet no CoD clone (in popularity geez) has spawn in the mean time. The thing is that the code to drive the logic is just a piece of the puzzle, having the gigabytes of F/OSS textures and models is the problem, hence the likes of OpenArena which took a while to get where they are.